Algebraic Geometry 4: A short note on Projective Varieties

What is a variety? It is the set of common zeroes for a set of polynomials. For example, for the set of polynomials \{x+y,x-y\}\in\Bbb{R}[x,y], the variety is (0,0).

Now what is a projective variety? Simply put, it is the common set of zeroes of polynomials in which a one-dimensional subspace is effectively considered one point. Hence, for the variety to be well-defined, if one point of a one-dimensional subspace satisies the variety, every point of the one-dimensional subspace has to satisfy that variety. Confused?

Take the polynomial x+y+z\in\Bbb{C}[x,y,z]. The point (1,-1,0) satisfies this polynomial. Now note that the points \lambda(1,-1,0) also satisfy this polynomial for every \lambda\in\Bbb{C}. Hence this is a projective variety. Now take x+y+z-1\in\Bbb{C}[x,y,z]. Here \lambda(1,0,0) satisfies the polynomial for only \lambda=1. Hence, this is not a projective variety.

But why? Why would you want to consider a whole line as one point? When you watch the world from your little nest, every line running along your ine of sight becomes a point. Hence, athough it may be a line in “reality” (whatever this means), for you it is a point. This is the origin of projective geometry, although things have gotten sightly complicated since then.

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Graduate student

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